The present invention relates generally to vehicular suspension systems, and more particularly relates to liquid springs utilized in such suspension systems.
In the past, various proposals have been made for replacing the conventional hydraulic shock absorber and exterior coil spring assemblies in vehicular suspension systems with more compact devices commonly referred to as liquid springs. A conventional liquid spring generally comprises a cylindrical housing having an internal chamber with a compressible liquid therein, a piston reciprocably disposed in the chamber and axially dividing it into bounce and rebound subchambers, a rod structure secured to the piston and axially movable into and out of the chamber, the rod structure having an external portion projecting outwardly from one of the housing ends.
With the liquid spring operatively interconnected between the vehicle frame and an associated wheel support structure, the compressible working liquid generates both spring and damping forces in the suspension system in response to relative axial translation between the rod structure and housing portion of the liquid spring in response to relative axial translation between the rod structure and the housing created by relative vertical displacement between the wheel structure and the frame.
The compressible working liquid permits the suspension system to exhibit a non-linear force/deflection curve. This non-linear force/deflection relationship is quite advantageous because it provides lower spring rates during normal operation and higher spring rates when the vehicle strikes a bump. Non linearity in the liquid spring-based suspension system is achieved via the compressibility of the silicone-based working liquid, which is approximately 1.5 to 2 times more compressible than conventional hydraulic fluid. In operation, the non-linear suspension system provides a spring rate which increases exponentially when the rod structure is moved from its normal static position relative to the cylinder housing to its fully retracted position within the cylinder housing. This increased spring force capability permits the liquid spring-based suspension system to easily absorb sharp bumps without bottoming out. In contrast, if the spring rate was linear, a substantially greater piston stroke would be required to enable the suspension system to absorb comparable bounce forces.
Conventional liquid spring suspension systems can be tuned and readily adjusted for differing vehicle weights (using the spring force of the liquid springs used in the suspension system), and for differing ride characteristics (using the damping forces of the liquid springs). It will thus be appreciated that liquid springs, via their utilization of a compressible working liquid to create both spring and damping forces, have a potential for significantly improved suspension performance compared to conventional hydraulic shock absorbers and associated exterior coil spring assemblies commonly used in vehicular suspension systems.
In an attempt to fully realize this potential, various improvements and refinements to the basic liquid spring structure described above have heretofore been proposed. One such proposed improvement, illustrated and described in U.S. Pat. Nos. 4,735,402 and 4,877,222, has been to provide what may be termed differential damping between the bounce and rebound strokes of the liquid spring - i.e., causing the damping force of the liquid spring in its bounce mode to be less than the damping force in its rebound mode.
Under this concept, as illustrated in these two patents, a flow passage system is extended generally axially through the piston and intercommunicates the bounce and rebound subchambers. Accordingly, when the piston moves through its bounce stroke liquid in the bounce chamber is forced into the rebound chamber through the piston passageway system, and when the piston moves through its rebound stroke liquid in the rebound chamber is forced into the bounce chamber through the passageway system. To provide the desired bounce/rebound damping force differential, a damper valve is slidably mounted on the rod structure between the piston and the cylinder housing end closure member through which the rod structure slidably and outwardly extends. The valve cooperates with the piston passageway system to restrict liquid flow through the passageway system in a manner such that the resistance to liquid flow from the bounce subchamber into the rebound subchamber is less than the resistance to liquid flow from the rebound subchamber into the bounce subchamber.
While as a general proposition this differential damping valve is suitable for its intended purpose, due to its positioning within the liquid spring it is unavoidably subjected to high mechanical impact forces that tend to wear away its critical sealing surface portions thereby creating valve leakage and resulting undesirable deviation from the intended directional flow resistance differential provided by the valve. This mechanical impact to which the damper valve is subjected arises from its additional use as a sealing and pressure relief structure which, as the piston is driven to its rebound limit position, enters and bottoms out within a complementarily configured recess in the closure member to protect the rod seal structure from liquid pressure damage and resulting leakage. Because of this positioning and additional use of the slidable damper valve illustrated in U.S. Pat. Nos. 4,735,402 and 4,877,222, the damper valve is forcibly slammed between the piston and the closure member each time the piston is driven through its maximum rebound stroke, thereby accelerating valve wear and leakage as described above.
In view of these problems and disadvantages associated with this conventional damper valve, it is accordingly an object of the present invention to provide a liquid spring having an improved differential damping system which eliminates or at least substantially minimizes such problems and disadvantages.